1. Field of the Invention
The present invention relates to a lens barrel which can be commonly used with different kinds of TV cameras having filters of different thickness provided in front of an image pickup device.
2. Description of Related Art
There are two kinds of TV camera bodies, one for a monochrome TV and one for a color TV. The camera bodies for the monochrome TV and the color TV are provided with different light receiving elements and different filters, having different refractive indexes and/or thicknesses and provided in front of the respective light receiving elements. In the monochrome TV camera body, a glass cover for the light receiving element is used as a filter whose thickness is about 0.8 to 1.2 mm. On the other hand, in the color TV camera body, a low-pass filter is provided in addition to the glass cover. The low-pass filter is usually made of a laminated composite of a crystal birefringent plate and an infrared filter, and accordingly has a thickness of about 5 to 6 mm. In general, the filters are provided on the camera side, and accordingly, there is a difference of 4 to 5 mm in thickness between the monochrome TV camera body and the color TV camera body.
In order to absorb the difference in thickness to thereby make it possible to commonly use a lens barrel (referred to as a common lens barrel) for both the monochrome TV camera body and the color TV camera body, the monochrome TV camera body and the color TV camera body have different flange-focal distances, so that an object image can be formed on the respective image pickup devices. However, the color image and the monochrome image to be formed have different aberrations that must be eliminated in the common lens barrel. Among the aberrations, the spherical aberration is considered most important, since the spherical aberration varies depending on the difference in thickness of the filters, thus resulting in an adverse influence on the image quality on the image surface. To this end, in conventional common lens barrels, the main solution has been directed towards elimination of the spherical aberration, taking into account a balance between the monochrome TV camera and the color TV camera, so that the image quality is not influenced much by the spherical aberration. Namely, in designing an optical system of the common lens barrel, the filter thickness is assumed to be an intermediate value between the thickness of the filters of the monochrome TV camera body and the thickness of the filters of the color TV camera body, so that a positive spherical aberration and a negative spherical aberration occur at the formation of a color image and a monochrome image, respectively. However, this is not the best solution, since an optimal visual position inherent in the lens is not used. In particular, for a high resolution camera or a high definition TV in which the resolution and brightness of the lens must be increased, the optical efficiency of the conventional common lens barrel is too poor to satisfactorily increase the resolution and brightness of the lens. To solve this problem, there have been attempts to produce separate lens barrels for the color TV camera body and the monochrome TV camera body, respectively. To make it possible to apply a lens barrel which has been designed to suit a color TV camera body to a monochrome TV camera body, it is also known to attach a filter to the rear end of the lens barrel to absorb the difference in thickness of the filter(s) between the color TV camera body and the monochrome TV camera body, to thereby compensate for the aberrations. However, these solutions invite an increase in the number of components, thus resulting in an increase of the manufacturing cost and an uneasy operation of the lens barrel.
Moreover, in a fast lens having an f-number of F0.7 to F0.8, the difference in thickness of the filters between the TV camera bodies is serious, since the image quality is largely influenced even by a small difference of about 0.5 mm in the filter thickness. In general, the color TV camera bodies produced by different manufacturers have different filter thicknesses.